Firefox extension fails to inject client if client was built with make dev

## Problem
The Firefox extension fails to inject the `client/build/boot.js` script of a development client if the client was built with `make dev` as indicated [here](https://h.readthedocs.io/projects/client/en/latest/developers/developing/#running-the-client-from-the-browser-extension).

The error is that the injected script returned non-structured-clonable data, which is forbidden in Firefox (see [here](https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API/tabs/executeScript#Return_value)).

This does not happen if the client is built with `yarn build` instead.

Chrome extension works OK with both ways of building the client.

## Cause
The reason seems to be that the browserify boot bundle returns a function, which would not be structured clonable.

It doesn't happen when the client is built with `yarn build` because here the `NODE_ENV` environment variable is set to `production` which causes the bundle to be minified, and the boot bundle's IIFE (immediately invoked function expression) is converted to a function expression (`! function foo()`), which returns `false` instead.

## Solution
As suggested [here](https://stackoverflow.com/a/44774834/10879911), adding an `undefined;` line at the end of the bundle solves the problem.

I just noticed a similar solution was proposed here before (commit https://github.com/hypothesis/browser-extension/pull/41/commits/2674d22cbfda70627ba225f99af815a457f1ab19), but then [reverted](https://github.com/hypothesis/browser-extension/commit/f17401a0eb36de9ad28ca3c8b2ecd3c4d96b26f3#diff-b67911656ef5d18c4ae36cb6741b7965) because of a later change in the client.

@robertknight, do you recall what the syntax error was? I've just reapplied this fix and it seems to be working OK both in Chrome and Firefox.